A stiffening element to provide variable resistance to movement between a pair of members at a joint. The stiffening element comprises a filler with particles flowable in a bladder. A pressure source is coupled to the bladder to vary a pressure within the bladder and collapse the bladder. Collapsing the bladder varies a flow characteristic of the filler within the bladder, Varying the flow characteristics of the filler varies resistance of the bladder to movement of the bladder, and thus a pair of movable members and the joint.